The present invention relates generally to a regenerative brake-by-wire braking system that utilizes a vacuum booster. More specifically, the present invention relates to a braking system that compensates for low vacuum levels in the hydraulic brake booster due to the loss of a vacuum source.
In recent years, electric automobiles using electric motors for driving vehicle wheels have attracted increasing attention or interest from the viewpoint of control of air pollution and reduction of noise generated by motor vehicles. Such an electric automobile may utilize so-called regenerative braking. To effect the regenerative braking, the electric motor for running the vehicle is placed in a power-generating state by regulating or limiting the supply of electric power to the electric motor, so that rotary motion of the drive wheels connected to the electric motor is converted into electric energy and is thus recovered, i.e., reused as driving force. The conversion of the rotary motion of the drive wheels and the electric energy involves consumption of kinetic energy, which is considered equivalent to braking of the drive wheels.
The above described brake apparatus is controlled to start regenerative braking when a driver releases his or her foot from an accelerator pedal of a vehicle. If a brake pedal is not depressed when the accelerator pedal is released, the extent of regenerative braking corresponds to engine braking performed by a conventional automobile driven by an internal combustion engine, for example. In this case, the brake apparatus is controlled to perform a weak regenerative braking. If the brake pedal is depressed, the brake apparatus is generally controlled to generate regenerative braking force in accordance with the degree or depth of depression in the brake pedal.
A typical mechanical brake system for an electric automobile, utilizing regenerative braking, includes a brake pedal and a vacuum-type booster. The vacuum-type booster is actuated according to the depression depth of the brake pedal. The vacuum tank for supplying vacuum is connected to the vacuum-type booster and the vacuum tank is equipped with a pump motor for reducing the pressure within the tank. The brake operating force generated by the vacuum-type booster is converted into a brake fluid pressure in a master cylinder. The master cylinder has a pressure sensor associated therewith for detecting the brake pressure and transmitting the sensed pressure to a brake controller which in turn causes brake actuating members to be actuated to apply mechanical brake force to the drive wheels.
In many instances, the vacuum source for the brake system, whether for an electric vehicle, a hybrid electric vehicle, or a conventional automobile, may be disabled or shut down under certain conditions, including normal conditions as well as fault conditions, as is known in the art. In typical systems where the vacuum source (engine) is turned off under normal conditions, the vacuum source must be restarted to raise the vacuum level whenever the vacuum level in the brake booster decreases to a lower level. This requires the engine to be started prematurely and on a more frequent basis than is required, thereby decreasing the overall efficiency of the system. Further, when vacuum levels in conventional systems become low, the brake pedal becomes stiff and provides for poor user braking feel.
It is therefore an object of the present invention to provide a system that compensates for low vacuum levels in the vacuum source (engine) based upon the deceleration of the vehicle.
It is a further object of the present invention to provide a regenerative brake-by-wire braking system with increased overall system efficiency.
In accordance with the above and other objects of the present invention, a brake-by-wire braking system is provided. The braking system includes a vacuum source, a vacuum booster and a vacuum level sensor for measuring the level of vacuum in the vacuum booster. The vacuum source is in electrical communication with a brake controller so that the brake controller can turn the vacuum source on and off and monitor whether the vacuum source is in an on or off condition. The brake controller is also in electrical communication with the vacuum level sensor to receive information indicative of the vacuum level in the vacuum booster. This information allows the brake controller to determine whether the level of vacuum is normal, below a normal operating level, or is critically low. The controller is also in electrical communication with a hydraulic booster which compensates for low vacuum levels when the vacuum level sensor determines that the level of vacuum in the vacuum booster is below the normal operating level.
These and other features of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanied drawings and appended claims.